Electrical insulators commonly known as suspension insulators may be used individually, but usually form part of a string to support an electrical conductor from a supporting structure. Generally such a suspension insulator comprises two metal hardware members secured to opposite surfaces of a suitably contoured porcelain insulator shell, one hardware member being embedded by means of cement in a cavity in the porcelain insulator shell. An electrical insulator of this type is disclosed in U.S. Pat. No. 3,941,918. By this arrangement the metal hardware members are separated and insulated each from the other. The hardware members, typically an upper cap and a lower pin, each are secured to one of the opposite surfaces of the insulator shell usually by a layer of cement or other suitable material.
In order to improve performance of the electrical insulator, the insulator shell surface is coated presently with a semiconductive glaze. Upon energization of the porcelain or string of such insulators, the glaze coating carries a small current which reduces radio interference and corona susceptibility while simultaneously heating the shell surface, whereby the insulator shell performs better in various adverse environments. With the glaze coating, it is necessary to have electrical connection between the glaze coating and both metal hardware fittings. The electrical connection between the glaze and the metal hardware transfers leakage current from one insulator shell to the next.
One approach in the prior art for providing electrical connection between the glaze and the hardware has been to connect the pin to the insulator shell using Portland cement which has a conducting additive therein so as to make the cement electrically conductive. Said U.S. Pat. No. 3,941,918 proposed one type of conductive cement useful for this purpose; other conductive cements have been proposed in U.S. Pat. No. 3,903,349 and in United Kingdom Pat. Nos. 1,363,428; 1,363,429 and 1,398,306. These conductive cements, however, have not been without problems. Addition of conductive materials to Portland cement can reduce the strength of the cement. Passage of current through the conductive cement, which always contains some moisture, can produce electrochemical reactions, especially at a cap or a pin when it has a positive polarity. Such reactions may be a accompanied by volumetric expansions, resulting in cracking or expansive rupturing of the porcelain insulator shell, thereby rendering the insulator nonfunctional and useless electrically. In practice the more severe problems occur at the pin, because at the pin the expansion forces generated by the conductive cement result in a tensile stress in the porcelain which the porcelain is less suited to withstand. Conversely the expansion forces generated by the cement at the cap compress the porcelain and the porcelain is particularly resistant to such compression.
U.S. Pat. No. 3,903,349 discusses the prior art attempts at overcoming the disadvantages of using Portland cement. For example, it is known to bond and epoxide resin layer containing aluminum powder to the metal components and glazed surface layer to cover and seal the Portland cement. However, the reference clearly indicates that resistance to weathering is unsatisfactory.
It has been attempted also to install metallic conductors between the metal hardware and the semiconductive glaze coating of the porcelain insulator shell and this approach is discussed in U.S. Pat. No. 2,239,809. Such metallic conductors carry the current from metal to glaze, without passing it through the cement so consequent volumetric expansion in the cement is avoided. But these metallic connectors have been found to create additional unwanted effects and, therefore, they are unsatisfactory. Initially these metallic conductors performed well; however in a relatively short time, severe corrosion and/or erosion of the glaze was experienced at the narrow band of interface between the glaze and the metal connector. Such corrosion and/or erosion resulted in rapid loss of glaze conductivity and continuity with consequent loss of benefit of the semiconductive glaze coating, plus other problems such as electrical discharges at the damaged glaze area and localized heat shock to the porcelain.